Field Trial of Optical Fibre Cable-TV System Optical Fibre System for ...

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171 ANNA-KARIN HANSSON Public Telecommunications Division Telefonaktiebolaget LM Ericsson STEN JACOBSON Ericsson Fiber Optics AB Fig. 3a, top The subscriber unit. The channels selected for TV and stereo programs are indicated on the front panel. It also contains a window that allows the built-in IR receiver to receive the signal from the subscriber's remote control unit Fig. 3b, above The subscriber unit The unit contains the WDM couplers and their fibre connections (upper right), the optical receivers and demodulators for the TV and stereo programs, the control unit for remote control and exchange control and also an optical transmitter for the data channel Fig. 2 shows the block diagram of the demonstration set-up. The pulse-frequency modulated optical links used are modified versions of the Ericsson standard product ZAV103, which is an optical video links for the transmission of TV-signals (video and audio) via an optical fibre. See the adjacent box for data on PFM links for TV and stereo channels. The data channel serves to transmit control data from the subscriber's remote control unit to the internal control data bus in the exchange. The control data from the remote control unit, which has a symbol rate of 300 baud, is converted in several stages to a data stream of approximately 300 kbit/s in the optical link, and is subsequently converted into exchange data via a built-in RS 232 port in the exchange. The TV and stereo programs are transmitted to the subscriber by means of two optical channels in the forward direction of the WDM system, and the control data channel is transmitted to the subcentre in a third optical channel in the backward direction of the WDM system. The WDM system is described in greater detail below. Fig. 3 shows the completed subscriber unit. The WDM system Fig. 4 shows the optical three-channel WDM system developed for the field trial. In order to demonstrate that WDM can be a technically and economically viable alternative, care has been taken to use components which allow the performance requirements of the completed system to be met at the lowest possible cost. The light sources chosen are three commercially available LEDs, all working at wavelengths below 940 nm, i.e. the "first window". The detectors used are one PIN diode and two APDs, also commercially available. The method chosen for the WDM couplers requires fewer components and is more suitable for serial production than most other types of coupler. It should therefore also be more economic in large-scale production. The system permits transmission distances of up to 1.5km with a fibre attenuation of 3 dB/km and a maximum coupler loss of 12dB (corresponding to 3 dB per coupler in the system as shown in fig. 4). The main limiting factor is the output power of the LEDs. In the field trial the transmission distance is ca. 1 km while the coupler losses are well below 3dB per channel, leaving plenty of margin for the output power of the light emitting diodes. A more critical parameter, however, is the crosstalk between the channels, since the LEDs have a fairly wide spectrum and their centre wavelenghts are relatively close to each other. The sys- Fig. 2 Block diagram of the demonstration installation. Wavelength division multiplex and pulse-frequency modulated links are the main building blocks In the system. Only one optical fibre is needed per subscriber Video conference room Cable TV subscriber
172 Subscriber side BP-890 Controll lå TV and data , | [stereo lit; programs 730 87o 890 ' en9th X1 \2 \3 730 810 890 \1 \2 \3 , Wavelength 1 km Fig. 4 Three-channel WDM system for fibre-optic subscriber tines. The three channels are obtained by cascading two two-channel WDM couplers with different characteristics. Optical bandpass filters are used to improve the channel separation in the system Control] data 1 \1 PIN Stereo | I :-:•! \2 LED-810 nm Exchange side I BP-890 \3 LED-890 nm tern requires at least 20dB optical crosstalk attenuation, which together with the attenuation requirements puts stringent demands on the optical filters used in the system. For the data channel which uses the 770 nm wavelength, the receiver can be made extremely sensitive (in practice -70dBm), which means that the output power of the 730-nm LED can be reduced from the maximum of 30u. to 2\i without any problems. Thereby, the crosstalk from the data channel into the TV and stereo channels at 890 and 810nm is considerably reduced. The principle of the WDM coupler is shown in fig. 6. The figure shows the demultiplexing function, but the basic principle is the same also for the multiplexing and duplex signalling functions. Two optical signals of different wavelength arrive at the coupler on one and the same fibre. The GRIN lens (GRaded INdex lens) acts as a collimator, i.e. the rays from the fibre form a point on one of the end surfaces of the lens and are converted to a parallel beam at the other end. The interference filter between the lenses is chosen so that one wavelength is transmitted and the other is reflected Each one of the two signals is then focused separately into point sources and sent out on the appropriate fibre. 100 - Fig. 5 Optical spectra for the three LEDs. The LEDs have wide spectra and the channels are also very close together, which means that optical bandpass filters must be used, see fig. 4 •^HB 730 nm IH^BI 610 nm ^ H M 890 nm 700 750 800 850 900
Page 1 and 2: ERICSSON REVIEW 4 1985 Field Trial
Page 3 and 4: Trial of Optical Fibre Cable-TV Sys
Page 5 and 6: Head end (HC] 280 Mbit/s Farsta aut
Page 7 and 8: 158 Switching equipment Fig. 7 Futu
Page 9 and 10: 160 Fig. 10 The teleconference room
Page 11 and 12: GERHARD GOBL Public Telecommunicati
Page 13 and 14: Fig. 5, above left Two-channel A/D
Page 15 and 16: Digital input signals Digital outpu
Page 17 and 18: Power ZAV 280/4 has individual powe
Page 19: Wavelength Division Multiplexing fo
Page 23 and 24: Fig. 8 Transmission characteristics
Page 25 and 26: Most computer cabinets are 19" wide
Page 27 and 28: 178 Fig. 5 Modems in Series 7 Modem
Page 29 and 30: Computer Aided Production of Plasti
Page 31 and 32: Fig. 4 Product geometry displayed o
Page 33 and 34: Rectifier for Mobile Telephone Syst
Page 35 and 36: I OR | OFU OS I CMUP HlsHi W±±±h
Page 37 and 38: 188 Fig. 6 The rectifier with the o
Page 39 and 40: Fig. 9 Rectifier BMJ A01 001 with t
Page 41 and 42: The Renewal of the London Undergrou
Page 43 and 44: 194 Rickmansworth, and the Loughton
Page 45 and 46: 196 Fig. 5 Optical fibre cable rout
Page 47 and 48: 198 In planning the optical fibre n
Page 49 and 50: Fig. 9 The digital telephone DIAVOX
Page 51 and 52: 202 Fig. 11 Transmission equipment
Page 53 and 54: 204 Fig. 2 Automatic testing of mul
Page 55 and 56: 206 Relative occurrence for F Fig.
Page 57 and 58: Fig. 5 Field testing of a 140 Mbit/
Page 59 and 60: 210 Fig. 7 The system down time as
Page 61 and 62: MTBF (years) Equipment 2-5 Transmul

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